Method for producing fluorine-containing polymer

ABSTRACT

An object of the present invention is to provide a method for producing a fluoropolymer which produces a large number of particles with a small particle size using a specific non-fluorous compound having surfactant ability. The method for producing a fluoropolymer includes aqueous dispersion polymerization of a monomer including at least one fluoroolefin in the presence of a compound represented by the formula: CH 2 ═CHCH 2 —O—(C═O)CHXCH 2 COO(CH 2 ) n CH 3  (in the formula, X is H or SO 3 Y wherein Y is NH 4  or an alkaline metal; and n is an integer of 0 to 19.)

TECHNICAL FIELD

The present invention relates to a method for producing a fluoropolymer using a non-fluorous compound having surfactant ability.

BACKGROUND ART

Fluoropolymers are used as materials for various products in a wide range of industries including automotive industry, semiconductor industry, chemical industry, and paint industry due to their advantages such as prominent chemical resistance, solvent resistance, heat resistance, and stain resistance.

Fluoropolymers are produced by emulsion polymerization, suspension polymerization, or solution polymerization of fluoroolefins. In the case of emulsion polymerization, surfactants are typically used. As more surfactant is used, more polymer particles are produced in early stages of the emulsion polymerization, resulting in a higher polymerization rate and improved productivity of a fluoropolymer. However, too much surfactant often results in a fluoropolymer that is poor in properties such as water resistance. This problem has created a need for the development of production methods by which polymerization is efficiently carried out in the presence of a small amount of a surfactant without causing adverse effects on the properties of a resulting fluoropolymer.

In response to the above-mentioned situation, a method for producing a fluoropolymer using a linear aliphatic sulfonate surfactant has been proposed (Patent Document 1). This method aims to replace expensive ammonium perfluorooctanoate, which is commonly used in emulsion polymerization of fluoropolymers, with a linear aliphatic sulfonate surfactant. However, one disadvantage of this method is that it produces only a small number of particles.

Also, the following methods were proposed: production methods using an alkylphosphoric acid or an ester thereof as a non-fluorous surfactant (Patent Documents 2 and 3); and a method using a compound containing a quaternary carbon atom to which groups, such as phosphoric acid, sulfonic acid, and carboxylic acid groups, are attached (Patent Document 4).

However, the use of an alkylphosphoric acid or an ester thereof does not completely ensure sufficient levels in terms of the number of produced particles, the polymerization rate, the molecular weight of resulting polymers, the polymer concentration of dispersions, the polymerization temperature, the polymerization pressure, and the like. Besides, a further increase in particle production is required of the method using a compound containing a quaternary carbon atom to which groups, such as phosphoric acid, sulfonic acid, and carboxylic acid groups, are attached.

Patent Document 1: U.S. Pat. No. 6,512,063

Patent Document 2: US 2007/0032591

Patent Document 3: US 2007/0018783

Patent Document 4: WO 2005/063827

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a method for producing a fluoropolymer which produces a large number of particles with a small particle size using a specific non-fluorous compound having surfactant ability.

Means for Solving the Problems

The present invention provides a method for producing a fluoropolymer, which includes aqueous dispersion polymerization of a monomer including at least one fluoroolefin in the presence of a compound (1) represented by the following formula (1):

wherein X is H or SO₃Y wherein Y is NH₄ or an alkaline metal; and n is an integer of 0 to 19.

A fluorosurfactant having 6 or less carbon atoms may also be present.

In order to achieve good lightfastness, chemical resistance, and solvent resistance, the fluoroolefin for the polymerization is preferably at least one fluoroolefin selected from the group consisting of vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, and chlorotrifluoroethylene.

In particular, the production method is effective for polymerization of vinylidene fluoride, tetrafluoroethylene, and chlorotrifluoroethylene, and for polymerization of vinylidene fluoride and hexafluoropropylene.

The aqueous distribution polymerization is preferably emulsion polymerization or suspension polymerization.

Effects of the Invention

The present invention enables production of a large number of fluoropolymer particles with a small particle size by using a specific non-fluorous compound having surfactant ability.

MODES FOR CARRYING OUT THE INVENTION

The method for producing a fluoropolymer of the present invention includes aqueous dispersion polymerization of a monomer including at least one fluoroolefin in the presence of a compound (1) represented by the following formula (1):

wherein X is H or SO₃Y wherein Y is NH₄ or an alkaline metal; and n is an integer of 0 to 19.

The compound (1) represented by the formula (1) is preferably a compound of the formula in which n is 10 or 11.

The fluoroolefin for the polymerization in the production method of the present invention is not particularly limited and may be one or more fluoroolefins. Examples of the fluoroolefin include perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl ethers) (PAVEs), and those represented by the formulas:

non-perfluoroolefins such as chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), vinylidene fluoride (VdF), trifluoroethylene, trifluoropropylene, pentafluoropropylene, tetrafluoropropylene, and hexafluoroisobutene. Examples of PAVEs include perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinyl ether) (PPVE).

Functional group-containing fluoroolefin monomers may also be used. Specific examples of such functional group-containing fluoroolefins include compounds represented by the formula (2):

CX¹ ₂═CX²—(Rf)_(m)—Y¹   (2)

(in the formula, Y¹ is —OH, —COOH, —SO₂F, or —SO₃M² (M² is hydrogen, NH₄, or an alkaline metal), a carboxylate salt, a carboxyl ester group, an epoxy group, or a cyano group; X¹ and X², which may be the same or different, are each hydrogen or fluorine; R_(f) is a C1-C40 divalent fluoroalkylene group or a C1-C40 divalent fluoroalkylene group including an ether bond; and m is 0 or 1.

Specific examples thereof include those represented by the following formulas.

Examples of the non-perfluoroolefins include iodine-containing monomers such as iodides of perfluoro vinyl ethers (e.g. perfluoro(6,6-dihydro-6-iodine-3-oxa-1-hexene) and perfluoro(5-iodine-3-oxa-1-pentene) disclosed in JP 05-63482 B and JP 62-12734 A).

In the present invention, non-fluorous monomer(s) copolymerizable with the fluoroolefin may be used together.

In the present invention, the aqueous dispersion polymerization is carried out in the presence of the compound (1) (surfactant). The aqueous dispersion polymerization is, for example, emulsion polymerization or suspension polymerization. In particular, emulsion polymerization is suited because a large number of polymer particles with a small particle size are produced. Specifically, it is preferred that emulsion polymerization is carried out in early stages of seed polymerization, that is, for the production of seed particles because a larger number of particles are produced even if the amount of the surfactant in the resulting fluoropolymer product is the same.

For example, in the case of emulsion polymerization, the amount of the compound (1) is preferably 10 to 5000 ppm, and more preferably 20 to 4000 ppm relative to the total amount of water. The amount is further preferably 50 to 1000 ppm, and particularly preferably 100 to 700 ppm. If the amount of the compound (1) is less than 10 ppm, the surfactant ability is low, resulting in the production of a smaller number of particles; if the amount is more than 5000 ppm, the polymerization rate is likely to be low.

Although the compound (1) alone is enough to stably proceed the emulsion polymerization, other surfactant(s) may be used together.

Examples of other surfactants include fluorosurfactants and non-fluorous (hydrocarbon) surfactants (except for the compound of the formula (1)).

Examples of such fluorosurfactants include anionic fluorosurfactants. These are preferred in terms of polymerization stability.

Examples of anionic fluorosurfactants include known ones such as those disclosed in US 2007/0015864, US 2007/0015865, US 2007/0015866, US 2007/0276103, US 2007/0117914, US 2007/142541, US 2008/0015319, U.S. Pat. No. 3,250,808, U.S. Pat. No. 3,271,341, JP 2003-119204 A, WO 2005/042593, WO 2008/060461, WO 2007/046377, WO 2007/119526, WO 2007/046482, and WO 2007/046345.

Specific examples of fluorosurfactants usable together include F(CF₂)_(n)COOM, CF₃CF₂CF₂OCF(CF₃) CF₂OCF(CF₃)COOM, CF₃CF₂OCF(CF₃)CF₂OCF (CF₃)COOM, CF₃OCF(CF₃)CF₂OCF (CF₃)COOM, H(CF₂CF₂)₂CH₂OCF(CF₃)COOM, H(CF₂)_(m)COOM, C₆F₁₃CH₂CH₂SO₃M, F(CF₂CF₂)_(p)CH₂CH₂SO₃M, and F(CF₂CF₂)_(q)CH₂CH₂SO₄M (in the formulas, M is a monovalent cation; n is an integer of 2 to 5; m is an integer of 2 to 10; p is an integer of 2 to 10; and q is an integer of 2 to 10.)

Especially, fluorosurfactants having 6 or less carbon atoms, in particular, anionic fluorosurfactants having 6 or less carbon atoms are preferred because a liquid polymer product containing a fluoropolymer at a high concentration is produced as a stable dispersion.

Examples of non-fluorous (hydrocarbon) surfactants include hydrocarbon surfactants such as CH₃(CH₂)_(r)SO₃M, CH₃(CH₂)_(s)SO₄M, CH₃(CH₂)_(t)COOM, H(CH₂)_(u)COO(CH₂CH₂O)_(v)H, and (NaSO₃)CH((CH₂)_(w)CH₃)((CH₂)_(x)CH₃) (in the formulas, M is a monovalent cation; r is an integer of 2 to 16; s is an integer of 2 to 16; t is an integer of 2 to 16; u is an integer of 2 to 40; v is an integer of 2 to 45; and w+x=10 to 20.)

Regarding the amount of other surfactant(s) usable together, for example, in the case of emulsion polymerization, the total amount of the compound (1) and other surfactant(s) is preferably 10 to 5000 ppm, and more preferably 20 to 4000 ppm relative to the total amount of water. If the total amount of the compound (1) and other surfactant (s) is less than 10 ppm, the surfactant ability is low, resulting in the production of a smaller number of particles; if the total amount is more than 5000 ppm, the polymerization rate is likely to be low.

Other examples of surfactants usable together include reactive fluorosurfactants each including a fluorocompound having a radical polymerizable unsaturated bond and a hydrophilic group in the molecule. If such a reactive fluorosurfactant is present in the reaction system during the polymerization, the reactive fluorosurfactant may be incorporated as a part of the polymer chain in a resulting polymer.

Examples of such reactive surfactants include fluorocompoounds disclosed in JP 08-67795 A.

The polymerization temperature is not particularly limited and is optimally determined by considering the type of polymerization initiator(s). Too high a temperature tends to reduce the monomer density in a gas phase or may induce a chain branching reaction of a polymer. Consequently, a target copolymer may not be obtained. Too low a temperature leads to slow polymerization, possibly resulting in low productivity. The polymerization temperature is preferably 40° C. to 120° C., and more preferably 50° C. to 100° C.

The monomer may be continuously added or portions of the monomer may be successively added.

The polymerization initiator(s) may be oil-soluble peroxides but peroxycarbonates such as diisopropyl peroxydicarbonate (IPP) and di-n-propyl peroxydicarbonate (NPP), which are typical examples of oil-soluble initiators, have the problems that they have a risk of explosion, and are expensive, and that they tend to cause scale build-up on the side wall of a polymerization reactor during the polymerization reaction. In order to further reduce the compression set of fluoropolymers, it is preferable to use water-soluble radical polymerization initiator(s). Preferred examples of water-soluble radical polymerization initiators include ammonium salts, potassium salts, and sodium salts of persulfuric acid, perboric acid, perchloric acid, perphosphoric acid, and percarbonic acid. In particular, ammonium persulfate and potassium persulfate are preferred.

The amount of polymerization initiator(s) is not particularly limited. An amount that does not significantly reduce the polymerization rate or more (for example, a few parts per million relative to the water concentration) of initiator(s) is added at once in an early stage of the polymerization, in portions, or continuously. The upper limit of the amount is within a range where polymerization reaction heat can be removed from the surface of the equipment.

In the production method of the present invention, molecular weight regulator(s) and the like may also be added. The molecular weight regulator(s) may be added at once in an early stage, or may be added continuously, or in portions.

Examples of molecular weight regulators include esters such as dimethyl malonate, diethyl malonate, methyl acetate, ethyl acetate, butyl acetate, and dimethyl succinate. Other examples thereof include isopentane, isopropanol, acetone, various mercaptans, carbon tetrachloride, cyclohexane, monoiodo methane, 1-iodomethane, 1-iodo-propane, isopropyl iodide, diiodo methane, 1,2-diiodomethane, and 1,3-diiodopropane.

In addition, other agents such as buffer may also be added optionally. In this case, it is preferred that the amount thereof is determined not to impair the effects of the present invention.

The polymerization pressure is suitably determined within the range of 0.1 to 10 MPa, and more suitably within the range of 0.2 to 8 MPa. Within the above ranges, the pressure may be low (0.1 to 1 MPa) or high (1 to 10 MPa).

As stirring means, for example, an anchor blade, a turbine blade, an inclined blade, or the like can be used, but stirring is preferably conducted by a large-scale blade such as FULLZONE or MAXBLEND in order to disperse monomers well and achieve good polymer dispersion stability. The stirring device may be a horizontal stirring device or a vertical stirring device.

The production method of the present invention can stably provide even a liquid polymer product containing a fluoropolymer at a high concentration. For example, if the compound (1) is used alone, a stable fluoropolymer dispersion having a fluoropolymer concentration of about 45% by mass is provided; if a fluorosurfactant, in particular, an anionic fluorosurfactant having 6 or less carbon atoms is used together, a stable fluoropolymer dispersion having a high fluoropolymer concentration of higher than 45% by mass, for example, 45 to 55% by mass is provided.

EXAMPLES

The following examples are set forth to specifically illustrate the present invention and are not intended to limit the present invention.

The devices and conditions used for evaluation of properties are listed below.

(1) Average Particle Size

Measuring device: MICROTRAC UPA available from Honeywell

Measuring method: Dynamic light scattering method

A sample was prepared by diluting an emulsion to be measured with pure water to a measurable concentration and measured at room temperature. An average size determined from data was regarded as a particle size.

(2) Number of Particles

Calculation Method: The number of particles was calculated from the average particle size determined in (1) and the solids content, assuming the specific gravity of each polymer to be 1.8.

(3) NMR Analysis:

NMR measuring device: product of VARIAN Inc.

¹H-NMR analysis condition: 400 MHz (tetramethylsilane=0 ppm)

¹⁹F-NMR analysis condition: 376 MHz (trichlorofluoromethane=0 ppm)

(4) Molecular Weight Analysis:

The standard polystyrene equivalent weight average molecular weight and number average molecular weight were determined using Shodex GPC-104 available from Showa Denko K. K.

Measurement Condition

Carrier: Tetrahydrofuran (THF)

Flow rate: 0.6 ml/min

Column temperature: 40° C.

Sample: 3% THF solution of resin to be measured

(5) Melt Flow Rate (MFR) Value

Dynisco melt-index tester available from YASUDA SEIKI SEISAKUSHO, LTD. was used. About 6 g portion of resin was fed into a 0.376-inch ID cylinder controlled at 250° C.±0.5° C., left to stand therein for 5 minutes for temperature equilibration, and then extruded through an orifice with a diameter of 0.0825 inch and a length of 0.315 inch under a piston load of 10 kg. (Each resin was sampled around the same time and measured three times. The average of the measurements was regarded as an MFR value. The unit is g/10 min.)

Example 1

A 2-L stainless steel autoclave was charged with 1000 g of ion exchange water and 1.32 g of a 38% by mass aqueous solution of a compound (1-1) represented by the formula (1-1):

(a mixture of those of the formula with n being 10 and 11) (compound (1-1) concentration: 500 ppm/water (water as polymerization medium, hereinafter the same)). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE/CTFE (=74/14/12 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.58 to 0.63 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 2.0 g (2000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 1.50 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, the monomer mixture VdF/TFE/CTFE (=74/14/12 mol %) was supplied to maintain the internal pressure within the range of 0.58 to 0.63 MPa. Unreacted monomers were discharged 2 hours 45 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 8.0% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE/CTFE=70.4/12.8/16.8 (mol %). The average particle size of the obtained fluoropolymer was 50 nm and the number of particles in the dispersion was 4.98×10¹⁴ (per g of water).

A 200 g portion of this dispersion was frozen at −10° C. for 24 hours to cause flocculation. The obtained flocs were rinsed with water and dried. In this manner, the fluoropolymer was recovered. The number average molecular weight (Mn) and the weight average molecular weight (Mw) of the fluoropolymer determined by GPC were 3.89×10⁴ and 6.57×10⁴, respectively, and the molecular weight distribution Mw/Mn was 1.69.

Comparative Example 1

A 2-L stainless steel autoclave was charged with 500 g of ion exchange water and 0.1 g (200 ppm/water) of a compound represented by the formula:

NaSO₃CH((CH₂)_(m)CH₃)((CH₂)_(n)CH₃)

(a mixture of those with m+n being 14 to 17). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE/CTFE (=74/14/12 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.60 to 0.65 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.0 g (2000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 0.75 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, the monomer mixture VdF/TFE/CTFE (=74/14/12 mol %) was supplied to maintain the internal pressure within the range of 0.60 to 0.65 MPa. Unreacted monomers were discharged 3 hours 7minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 10.2% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE/CTFE=72.2/12.8/15.0 (mol %). The average particle size of the obtained fluoropolymer was 160.8 nm and the number of particles in the dispersion was 2.90×10¹³ (per g of water).

A 200 g portion of this dispersion was frozen at −10° C. for 24 hours to cause flocculation. The obtained flocs were rinsed with water and dried. In this manner, the fluoropolymer was recovered. The number average molecular weight and the weight average molecular weight determined by GPC were 7.81×10³ and 7.54×10⁴, respectively, and the molecular weight distribution Mw/Mn was 9.65.

Comparative Example 2

A 2-L stainless steel autoclave was charged with 500 g of ion exchange water and 0.50 g of a 50% aqueous solution of ammonium perfluorooctanoate (ammonium perfluorooctanoate concentration: 1000 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE/CTFE (=74/14/12 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 1.00 to 1.10 MPa. The temperature was raised to 60° C.

Then, a polymerization initiator solution prepared by dissolving 0.20 g (400 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water was introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, the monomer mixture VdF/TFE/CTFE (=74/14/12 mol %) was supplied to maintain the internal pressure within the range of 1.00 to 1.10 MPa. Three hours later, 0.20 g (400 ppm/water) of APS was introduced under pressure of nitrogen gas. Unreacted monomers were discharged 8 hours after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 25.5% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE/CTFE=78.5/11.4/10.1 (mol %). The average particle size of the obtained fluoropolymer was 115 nm and the number of particles in the dispersion was 2.23×10¹⁴ (per g of water).

A 200 g portion of this dispersion was frozen at −10° C. for 24 hours to cause flocculation. The obtained flocs were rinsed with water and dried. In this manner, the fluoropolymer was recovered. Since this fluoropolymer was insoluble in THF, GPC to determine its molecular weight could not be performed.

Example 2

A 2-L stainless steel autoclave was charged with 1000 g of ion exchange water and 0.52 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 200 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/HFP (=78/22 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.75 to 0.80 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.00 g (1000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 1.50 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, the monomer mixture VdF/HFP (=78/22 mol %) was supplied to maintain the internal pressure within the range of 0.75 to 0.80 MPa. Three hours later, 1.00 g (1000 ppm/water) of APS was introduced under pressure of nitrogen gas. Unreacted monomers were discharged 5 hours 15 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 10.2% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/HFP=88.2/11.8 (mol %). The average particle size of the obtained fluoropolymer was 50 nm and the number of particles in the dispersion was 9.64×10¹⁴ (per g of water).

A 200 g portion of this dispersion was frozen at −10° C. for 24 hours to cause flocculation. The obtained flocs were rinsed with water and dried. In this manner, the fluoropolymer was recovered. The number average molecular weight and the weight average molecular weight determined by GPC were 1.15×10⁴ and 5.70×10⁴, respectively, and the molecular weight distribution Mw/Mn was 4.96.

Example 3

A 6-L stainless steel autoclave was charged with 2675 g of ion exchange water and 1.41 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 200 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE/CTFE (=71.5/14.9/13.6 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.75 to 0.8 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 5.35 g (2000 ppm/water) of ammonium persulfonate (APS) in 16 ml of ion exchange water and 2.14 g (800 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 500 rpm.

When the internal pressure began to decrease in the course of the polymerization, the monomer mixture VdF/TFE/CTFE (=71.5/14.9/13.6 mol %) was supplied to maintain the internal pressure within the range of 0.75 to 0.8 MPa. A solution of 1.41 g of the 38% by mass aqueous solution of the compound (1-1) in 4 ml of ion exchange water was introduced under pressure when monomer mixture consumption reached 669 g, 1146 g, 1440 g, and 1783 g, respectively. Unreacted monomers were discharged 14 hours 44 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 44.4% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE/CTFE=75.1/12.4/12.5 (mol %). The average particle size of the obtained fluoropolymer was 142 nm and the number of particles in the dispersion was 2.92×10¹⁴ (per g of water).

A 200 g portion of this dispersion was frozen at −10° C. for 24 hours to cause flocculation. The obtained flocs were rinsed with water and dried. In this manner, the fluoropolymer was recovered. The MFR measurement of this fluoropolymer resulted in 9.18 g/10 min.

Example 4

A 2-L stainless steel autoclave was charged with 2675 g of ion exchange water, 11.8 g of a 50% by mass aqueous solution of ammonium perfluorohexanoate (ammonium perfluorohexanoate concentration: 2200 ppm/water), and 1.41 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 200 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE/CTFE (=73.3/14.4/12.3 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.75 to 0.8 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 5.35 g (2000 ppm/water) of ammonium persulfonate (APS) in 16 ml of ion exchange water and 0.75 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 500 rpm.

When the internal pressure began to decrease in the course of the polymerization, the monomer mixture VdF/TFE/CTFE (=73.3/14.4/12.3 mol %) was supplied to maintain the internal pressure within the range of 0.75 to 0.80 MPa. Unreacted monomers were discharged 5 hours 18 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 45.5% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE/CTFE=75.7/12.4/11.9 (mol %). The average particle size of the obtained fluoropolymer was 142.4 nm and the number of particles in the dispersion was 3.07×10¹⁴ (per g of water).

A 200 g portion of this dispersion was frozen at −10° C. for 24 hours to cause flocculation. The obtained flocs were rinsed with water and dried. In this manner, the fluoropolymer was recovered. The MFR measurement of this fluoropolymer resulted in 2.34 g/10 min.

Example 5

A 2-L stainless steel autoclave was charged with 500 g of ion exchange water and 0.26 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 200 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, VdF was introduced under pressure to provide a system pressure in the polymerization reactor of 0.75 to 0.80 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.00 g (2000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 0.75 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, VdF was supplied to maintain the internal pressure within the range of 0.75 to 0.80 MPa. Unreacted monomers were discharged 8 hours 40 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a VdF homopolymer (PVdF) dispersion (solids content: 3.4% by mass) was obtained.

The average particle size of the obtained PVdF was 330 nm and the number of particles in the dispersion was 1.04×10¹² (per g of water).

Example 6

A 2-L stainless steel autoclave was charged with 500 g of ion exchange water and 0.26 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 200 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE/HFP (=50/20/30 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.55 to 0.60 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.00 g (2000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 0.75 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, a monomer mixture VdF/TFE/HFP (=50/20/30 mol %) was supplied to maintain the internal pressure within the range of 0.55 to 0.60 MPa. Unreacted monomers were discharged 2 hours 30 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 8.6% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE/HFP=60.9/25.8/13.3 (mol %). The average particle size of the obtained fluoropolymer was 50.2 nm and the number of particles in the dispersion was 7.89×10¹⁴ (per g of water).

Example 7

A 2-L stainless steel autoclave was charged with 1000 g of ion exchange water and 1.04 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 400 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, VdF was introduced under pressure to provide a system pressure in the polymerization reactor of 2.3 to 2.5 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.0 g (1000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water was introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 300 rpm.

When the internal pressure began to decrease in the course of the polymerization, VdF was supplied to maintain the internal pressure within the range of 2.3 to 2.5 MPa. APS (500 ppm/water) was added 5.5 hours and 7 hours after the start of polymerization, respectively. Unreacted monomers were discharged 9 hours 30 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a VdF homopolymer (PVdF) dispersion (solids content: 10.3% by mass) was obtained.

The average particle size of the obtained fluoropolymer was 85.6 nm and the number of particles in the dispersion was 1.94×10¹⁴ (per g of water).

Example 8

A 2-L stainless steel autoclave was charged with 500 g of ion exchange water and 0.26 g of a 38% by mass aqueous solution of the compound (1-1) (compound (1-1) concentration: 200 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, a monomer mixture VdF/TFE (=60/40 mol %) was introduced under pressure to provide a system pressure in the polymerization reactor of 0.75 to 0.80 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.00 g (2000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 0.75 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 600 rpm.

When the internal pressure began to decrease in the course of the polymerization, a monomer mixture VdF/TFE (=60/40 mol %) was supplied to maintain the internal pressure within the range of 0.75 to 0.80 MPa. Unreacted monomers were discharged 2 hours 35 minutes after the start of polymerization, and then the autoclave was cooled. In this manner, a fluoropolymer dispersion (solids content: 12.0% by mass) was obtained.

The copolymer composition determined by NMR analysis was VdF/TFE=64.2/35.8 (mol %). The average particle size of the obtained fluoropolymer was 71.4 nm and the number of particles in the dispersion was 3.98×10¹⁴ (per g of water).

Comparative Example 3

A 2-L stainless steel autoclave was charged with 1000 g of ion exchange water and 0.50 g of a 50% aqueous solution of ammonium perfluorohexanoate (ammonium perfluorohexanoate concentration: 1000 ppm/water). The system was sufficiently purged with nitrogen gas, and then depressurized. Subsequently, VdF was introduced under pressure to provide a system pressure in the polymerization reactor of 0.75 to 0.80 MPa. The temperature was raised to 70° C.

Then, a polymerization initiator solution prepared by dissolving 1.0 g (1000 ppm/water) of ammonium persulfonate (APS) in 4 ml of ion exchange water and 0.75 g (1500 ppm/water) of ethyl acetate were introduced under pressure of nitrogen gas. Thus, the reaction was initiated while stirring at 300 rpm.

Although this condition was kept for approximately 7 hours, no VdF polymer was produced. 

1. A method for producing a fluoropolymer, comprising aqueous dispersion polymerization of a monomer including at least one fluoroolefin in the presence of a compound (1) represented by the following formula (1):

wherein X is H or SO₃Y wherein Y is NH₄ or an alkaline metal; and n is an integer of 0 to
 19. 2. The production method according to claim 1, wherein a fluorosurfactant having 6 or less carbon atoms is also present.
 3. The production method according to claim 1, wherein the fluoroolefin is at least one fluoroolefin selected from the group consisting of vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, and chlorotrifluoroethylene.
 4. The production method according to claim 1, wherein the fluoroolefin is vinylidene fluoride, tetrafluoroethylene, and chlorotrifluoroethylene.
 5. The production method according to claim 1, wherein the fluoroolefin is vinylidene fluoride and hexafluoropropylene.
 6. The production method according to claim 1, wherein the aqueous dispersion polymerization is emulsion polymerization 